Distributed, peer to peer mail.
That sounds like something from a sci-fi novel.

I can see it now - you sign up and log where/when you're going, and how much space you have to take packages/letters with you. Some algorithm matches your as a courier up with people who need stuff taken where you're going.

The service is free if you act as the courier (...or mule, I can totally see that being misused rapidly).

I don't know how you get around the depot and 'guy who goes door to door' model though.

Oh! Maybe that's a good startup idea - some kind of 'mail is here to be picked up' notification for the USPS, which could then optimize it's route and only hit places it had mail to deliver/pick up from.
I go days with no in or outgoing snail mail, but the poor postman has to walk up to my mailbox and check (no flag, it's inside the building).